The Hazardous and Solid Waste Amendments (HSWA) to the Resource Conservation and Recovery Act (RCRA), United States Public Law P.L. 98-616 include specific provisions restricting the direct land disposal of many hazardous wastes, including contaminated soils. These restrictions, commonly termed the "Land Ban" require that many types of waste be treated to reduce the toxicity or mobility of the hazardous components. Similar provisions have been incorporated into regulations under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) of 1980 which is commonly referred to as "Superfund" and in the Superfund Amendment and Reauthorization Act of 1986 Regulations written by the United States Environmental Protection Agency (EPA) under both acts specify methods for treating a variety of wastes prior to land disposal.
The methods specified for wastes containing organic contaminants is incineration or thermal treatment. The specified method for wastes containing metals is solidification/stabilization. Wastes containing both organics and metals, such as from the production or use of metalo-organic compounds, are difficult to incinerate. The temperatures encountered in incineration volatilize the metals and their compounds resulting in the formation of metal fumes which are very fine and which require very complex and energy intensive air pollution control equipment.
An alternative method of treating contaminated solids, especially contaminated soils is indirect thermal treatment. It is proving to be the thermal treatment method of choice for many applications. Indirect thermal treatment is differentiated from incineration by two features. First, the temperatures tend to be lower (on the order of 85.degree. to 385.degree. C. rather than typically 535.degree. C. and up for incineration) and second, the waste is not directly exposed to the flame so that the contaminants released from the material being treated are not mixed with the combustion gases from the flame. Separating the waste from the combustion gases reduces the volume of contaminated gases that need to be cleaned and reduces the likelihood of formation of undesirable products of combustion such as chlordibenzodioxins and chlorodibenzofurans.
Indirect thermal treatment systems presently in use overcome many of the problems encountered when attempting to incinerate contaminated solids such as soils but they are based on equipment much like that used for incinerators. Examples of such equipment include the rotary kilns disclosed in the U.S. Pat. No. 4,864,942 issued to Fochtman et. al. and in the U.S. Pat. Nos. 4,782,625 and 4,951,417 issued to Gerkin et. al., the fluidized beds disclosed in U.S. Pat. Nos. 4,778,606, 4,685,220, 4,463,691, and 4,699,721 issued to Meenan et. al., and the pyrolitic chambers disclosed in U.S. Pat. Nos. 4,306,961, 4,180,455, and 4,280,879 issued to Taciuk. Noland recognizes in U.S. Pat. No. 4,738,206 the inherent problems in the use of incinerator-style hardware but his system utilizes an archimedes screw device for heating and conveying soils through the thermal desorber. Many disadvantages exist with such equipment. Screw devices are very complex and prone to jamming on large or sticky materials. Kilns and fluidized bed systems require relatively large amounts of gas. The rotary kilns are difficult to seal tightly and to prevent undesired air infiltration and fluidized bed systems require substantial amounts of gas to fluidize the solids. Fluidized beds cannot treat rocks, debris and other materials which cannot be fluidized. Because of these limitations, present systems produce significant amounts of particulate which is usually hazardous and which requires large and highly efficient air pollution control devices.
Indirect heating is a relatively inefficient method of transferring heat to a solid. Typically, the heat is transferred by contact between the solid and the walls of the vessel containing the solid and by blowing a hot, inert gas (termed herein as "purge gas") over the solids. With the exception of the system disclosed by Noland, the heat transfer occurs in a kiln, a device with a relatively small heat transfer area. The problem is typically overcome by also passing hot gases through the system to improve the heat transfer efficiency. Gases that have been used are nitrogen, air, and combustion gases from the indirect heater.
In the paper by Fox, R. D., Alsperin, E. S., & Huls, H. H. "Thermal Treatment for the Removal of PCBs and Other Organics from Soil" Environmental Progress (Vol. 10, No1) February, 1991, P.40, thermal separation is discussed through the use of indirect heating of the contaminated material in a rotating metal chamber. The contaminants are separated by volatilization with the contaminants then being collected through the use of condensation techniques.
In the paper by dePercin, Paul, "Thermal Desorption Attainable Remediation Levels" Remedial Action Treatment and Disposal of Hazardous Waste, Proceedings of the Seventeenth Annual RREL Hazardous Waste Research Symposium, Environmental Protection Agency, EPA/600/9-91/002, April 1991 P.511 (Available from the Risk Reduction Engineering Laboratory, EPA, Cincinnati, Ohio) there is discussed one system (Described in Taciuk's patents) which does not use heated gas. Rather, the soil is simply heated to a temperature on the order of 535.degree. C. and the organics are allowed to pyrolyze. This is a very high temperature approaching that of incineration. Such a high temperature is necessary because the process does not use a purge gas. Systems which utilize purge gases can operate at lower temperatures than those that do not because the inert purge gases sweep the organics away from the solids as they are desorbed.
The purge gas is, therefore a critical element of a low temperature desorption system but it results in significant volumes of gas that must be treated prior to discharge. All processes in operation at present utilize a non-condensible gas for purging. Gases used are: air, nitrogen, carbon dioxide, or vent gas from the auxiliary combustor. The non-condensible gases have the following two major disadvantages:
1. The condensible gases (water, contaminants) removed from the contaminated solids are diluted by the non-condensible purge gas reducing the condensers' efficiency to remove the organics. As a result, thermal treatment systems must use refrigerated condensers, and relatively large adsorbers to capture the contaminants released from the solids.
2. The desorption process releases relatively large amounts of contaminated particulate matter. As a result, the off-gas stream must be treated to control it. The use of condensible purge gases reduce the amount of gas requiring treatment; hence, the size and cost of the requisite air pollution control devices is decreased.
Thermal desorption systems currently in use cannot use condensible purge gases because it is virtually impossible to exclude air from rotating systems that have substantial gas flows inside them. The present invention utilizes a specially designed non-rotating contactor with very few penetrations, which are small and can be readily sealed, to essentially eliminate air infiltration or releases of contaminated vapors and to thus make it possible to use superheated steam as a purge gas.
Finally, present thermal desorption systems cannot stabilize materials such as soils contaminated with metals. It is necessary to treat residues containing metals with binders in a second piece of apparatus in order to stabilize the product and reduce the leachability of metals present. Stabilization/solidification is the process whereby contaminated materials are mixed with a binder such as portland cement, lime, lime-kiln flyash, quicklime, cement kiln flyash, coal flyash or other, similar, materials either individually or in combination. Sometimes additives such as soluble silicates or iron compounds are added to change the process's characteristics. The resulting product typically sets up into a monolithic or granular material. The contaminants are bound and made less mobile by the process. Solidification/stabilization (S/S) is a well established treatment technology which has been used for many years to improve the handling and physical characteristics of soils and sludges and to reduce the mobility of toxic and carcinogenic metals. The technology does not work as well when the wastes being stabilized contain organics. Organics can reduce the strength and increase the leachability of the treated material and, as shown by Weitzman the act of stabilizing/solidifying vaporizes a substantial fraction of the volatile and semi-volatile organics contaminants.
Typically, substantial amounts of heat are generated during the mixing and solidification of the contaminated materials and binders. In fact, temperatures of 100.degree. C. and greater can occur. In the paper by Weitzman, Leo "Volatile Emissions From Stabilized Wastes" Proceedings of the Fifteenth Annual RREL Hazardous Waste Research Symposium, Environmental Protection Agency, EPA/600/9-90/006, February, 1990 P.448. (Available from the Risk Reduction Engineering Laboratory, EPA, Cincinnati, Ohio), there is described the release of up to 70% of organic contaminants during the mixing of binder and contaminated soils. Such release of organics is undesirable when the mixing occurs in the open, as is common. The present invention takes advantage of the phenomenon. By mixing the binder prior to thermal desorption, the heats of reaction of the solidification/stabilization process are utilized to reduce the fuel requirement for the desorption system.
The present invention takes advantage of the tendency for the solidification/stabilization process to volatilize organics from solids and to enhance the process by combining it with thermal treatment. My invention:
1. Simplifies the soil heating system while improving the system's heat transfer characteristics.
2. Allows the system to be readily sealed against air infiltration so that it can be operated with a variety of sweep gases, including, superheated steam. This feature minimizes the volume of the gas streams that need to be moved and allows the design to be highly compact for ease of transport. This feature also minimizes fine particulate (fume) formation which, combined with the very low volumes of gas that need to be treated makes the technology especially suitable for removing organic contaminants from solids contaminated with toxic metals such as arsenic, and lead.
3. Allows the system to be operated at any combination of temperature and sweep gas flow rates to optimize performance on different types of contaminated materials.
4. Allows the use of a condensible gas, superheated steam as the sweep gas for the system. Since non-condensible gases significantly impair the ability of condensers to remove organic constituents, this improves the efficiency of condensers and makes them a viable pollution control device for a thermal desorption system. Adsorbers are required to treat a much lower volume gas stream consisting of the small amount of remaining non-condensible gas.
5. Allows the contaminants removed from the soil to be concentrated in an aqueous phase or a gas phase, depending on the nature of the contaminant and how it can best be destroyed subsequent to removal from the solids.
6. Allows the system to be very compact so that complete modules of commercially viable units can be mounted directly on one or a few over-the-road vehicles or trailers and still remain within the U.S. Department of Transportation size limitations. Setup at the site require connecting a relatively few ducts, hoses and power lines with little or no field assembly required. This feature reduces the cost of transport, set-up and knockdown at a contaminated site.
7. Allows a high heat transfer area per unit area occupied. A compact system can, therefore have a very high solids retention time, since for a given soil, contaminant removal increases with higher temperature and with longer residence time. The compact design allows the use of lower temperatures for improved energy efficiency and a reduced likelihood of vaporizing substances such as toxic metal salts.
8. Takes advantage of the heat released by many types of binders when mixed with soil, sludge, dredging material and other types of solids to help remove the organics from the soil.